cos(a/2)不等于0.
tan(a/2) = sin(a/2)/cos(a/2) = 2sin(a/2)cos(a/2)/{2[cos(a/2)]^2}
= sin(a)/[1 + cos(a)]
当 sin(a/2)不等于0时,
tan(a/2) = sin(a/2)/cos(a/2) = 2[sin(a/2)]^2/[2sin(a/2)cos(a/2)]
= [1 - cos(a)]/sin(a)